1. Field of the Invention
The present invention relates to a miniaturized, thin semiconductor device and a manufacturing method thereof.
2. Description of Related Arts
In the effort of reducing a semiconductor chip in thickness, there has been performed a backside grinding process for grinding the back surface side of a semiconductor wafer (hereinafter, referred to simply as the wafer) on which elements and wiring are already formed. The backside grinding process is generally performed by laminating a soft protection film on the surface of the wafer, and then by rotating the wafer while pressing the back surface of the wafer against a grinding wheel with pressure being applied to the wafer through the protection film.
However, during the processes subsequent to the grinding, for example, the dicing process for cutting individual chips away from the wafer and the mounting process for mounting the diced chips onto a lead frame, the wafer and the chips are handled by a robot. Hence, if a reduction in thickness is pursued excessively, breaking occurs in the wafer and the chips when handled, and as a consequence, the yield is lowered. In particular, the diameter of today's wafer is on the increase, and a thin wafer obtained through the backside grinding may brake easily.
In order to solve such a problem, for example, Japanese Laid-Open Patent Publication (KOKAI) No. 11-150090 (1999) proposes the use of a protection reinforcement plate, which is a resin film formed on the surface of the wafer after a group of protruding electrodes are formed on the surface of the wafer. According to the manufacturing method of a semiconductor device of this publication, the backside grinding is applied to the wafer after the resin film is formed, and further, the top portion of the group of protruding electrodes is exposed by removing the surface layer portion of the resin film through etching. Subsequently, the resin film is removed along scribing lines, and further, a nitride film to be used as a passivation film is formed in a region avoiding the protruding electrodes, after which the wafer is cut along the scribing lines and divided into individual chips.
According to this method, the wafer after the backside grinding is reinforced by the resin film, and the individual chips cut away from the wafer are reinforced by the resin film as well. It is thus possible to handle the wafer and the chips in a satisfactory manner without causing any breaking. In addition, because mounting of the chips can be achieved by connecting the exposed top portion of the protruding electrodes to electrode pads on a wiring board, the semiconductor device can be reduced in thickness markedly in comparison with an arrangement such that an external terminals are drawn through wire bonding.
The above-described manufacturing method of the related art, however, has a problem that, as shown in FIG. 6 exaggeratedly, warping occurs in the wafer during a period since the resin film was formed on the surface of the wafer until the backside grinding is performed due to a difference in thermal expansion/contraction coefficient between the wafer and the resin film. Grinding a wafer having warping with a flat grinding wheel yields a difference in thickness between the central region and the edge region of the ground wafer, which makes it impossible to obtain semiconductor chips of a uniform thickness. Moreover, the semiconductor chips cut away from the edge portion of the wafer may not be reduced to a target thickness.
Meanwhile, the wafer reduced in thickness is laminated to an adhesive tape called a dicing tape, and then divided into individual pieces in the dicing process. However, because the wafer is cut by a thin grinding wheel (20 to 100 μm) called a blade during the dicing process, flaws called chipping or microcracking occur at the corners of the diced pieces.
The semiconductor substrate in the form of individual pieces is peeled off from the dicing tape by being pushed up with a needle from the back surface of the dicing tape, whereby perfect pieces are obtained. In this instance, however, the needle or the like damages the semiconductor substrate, and deteriorates the quality considerably. Also, the fabricated semiconductor device is exposed at the back surface on the side opposite to the active surface of the semiconductor substrate, and is therefore fragile and less reliable.
Incidentally, a wafer level CSP (Chip Size Package) is obtained by forming redistribution wiring and a sealing resin film sequentially on the active surface of the semiconductor chip on which circuits are formed. The redistribution wiring includes electrode pads, and posts (pillar-shaped electrodes), made of copper (Cu) and penetrating through the resin film, are formed on the electrode pads. A bump material, such as a solder ball, is bonded to the tip end of each post, so that the post can be bonded to the electrode pad or the like on the wiring board through the bump material.
The wafer level CSP is obtained by completing the processes from the formation of the circuits to the formation of the bump material on a semiconductor wafer containing a number of regions corresponding to individual semiconductor chips, and then cutting the semiconductor wafer into individual semiconductor chips. Consequently, the size of the semiconductor device (wafer level CSP) is almost as small as the size of the semiconductor chip.
However, in a case where the semiconductor substrate, such as a semiconductor wafer and a semiconductor chip, is made of silicon, for example, and the resin film is made of epoxy resin, for example, there is a considerable difference in thermal expansion coefficient between the semiconductor substrate and the resin film. Also, the posts made of copper exert considerable bonding strength with respect to the resin film. For these reasons, when the semiconductor substrate and the resin film undergo thermal expansion/contraction during the fabrication sequence or operations of the semiconductor device, the posts together with the resin film cause displacement, and considerable shearing stress is applied to a junction portion between the posts and the electrode pads, which may result in breaking of an electrical connection between the posts and the electrode pads.
In order to avoid such an unwanted event, stress applied to the posts needs to be dispersed by forming higher (longer) posts. However, in the case of forming the posts through plating, because a time needed for the plating is nearly proportional to the length of the posts, it takes a considerable time to form long posts, and as a result, a long manufacturing time is needed for a semiconductor device.
Further, because copper is an element that readily migrates, for example, in a case where the resin film is made of polyimide that readily absorbs moisture, if neighboring posts are formed too close, these posts electrical short-circuit due to migration. Hence, a wide space needs to be secured between neighboring posts, which is a factor that makes miniaturization of the semiconductor device difficult.